(1) Field of the Invention
This invention relates to a heating device including a heating member for contacting a medium to be heated, and a heating element mounted inside the heating member. This heating device is suited for fixing ink dispensed to printing paper in an inkjet printing apparatus, or fixing toner adhering to printing paper in an electrophotographic printing apparatus.
(2) Description of the Related Art
In recent years, plateless printing apparatus have been put to practical use, which do not require time to create plates, and yet are capable of high-speed printing while changing print contents as needed. Printing modes used in such plateless printing apparatus include an inkjet mode which forms print images by dispensing ink droplets directly to printing paper, and an electrophotographic mode which forms print images by transferring toner from an exposing drum to printing paper. Such a mode is employed as appropriate according to the purpose for which prints are used.
What is needed in a plateless printing apparatus, whether it employs the inkjet mode or the electrophotographic mode, is heating of the printing paper after printing is performed. Whether it is the inkjet mode or the electrophotographic mode, drying of ink droplets or fixation of toner by heating the printing paper after printing prevents scattering of the ink or toner to the internal structure of the plateless printing apparatus, or prevents damage to the printed image due to abrasion of the paper at the time of transportation.
The plateless printing apparatus which transports the printing paper at high speed employs a heating roller for heating such printing paper. The heating roller includes a heat source mounted inside such as a halogen lamp, sheathed heater, or electromagnetic inductor supported by a holding member for heating a roller body to heat printing paper transported so that the back surface of the paper having a print image formed thereon will contact the roller surface (see Japanese Unexamined Patent Publication No. 2015-1706, for example).
The heating roller used in the plateless printing apparatus is rotatable as driven by paper transportation, or by torque applied from a motor or the like. In order to realize smooth rotation, the heating roller has a rotary shaft thereof supported by bearings such as rolling bearings or plain bearings. Such bearings essentially require use of a lubricant in order to prevent damage to rolling elements such as balls or rollers or to the sliding plane.
In the case of an ordinary roller, the lubricant of the bearings can maintain its lubricating function for a relatively long period of time. However, in the case of the heating roller, the lubricant of the bearings will deteriorate in a relatively short period of time, since the heat from the roller heated by the heat source conducts to the rotary shaft to heat the rotary shaft to a high temperature.
Further, in order to prevent melting of an electrode at a heat source end supported by the holding member, the interior of the rotary shaft is ventilated to cool the heat source end. However, the hot air having been blown to the hot heat source end will heat the rotary shaft to cause a further temperature rise thereof when discharged as exhaust air from a gap between the holding member and the rotary shaft. As a result, the lubricant of the bearings deteriorates in a still shorter period of time.
Under such circumstances, the bearings of the heating roller have frequently been damaged since the lubricant can deteriorate at a very early stage. In order to prevent damage to the bearings, it is necessary frequently to supply the bearings with the lubricant, or to change the bearings per se. This has resulted in a frequent occurrence of situations necessitating a suspension of operation of the plateless printing apparatus.